The present invention relates to optical memory, more specifically to the optical memory medium and the storage method therewith which enables high-quality, high-density wavelength-multiplexing optical storage based on the photochemical hole-burning phenomenon at cryogenic temperature.
Castro, et al, proposed a wavelength multiplexing optical memory based on the photochemical hole-burning (PHB) phenomenon in 1978 (U.S. Pat. No. 4,101,976, 1978). Since then, much attention has been paid to PHB as a superhigh-density optical storage scheme that can principally realize more than 1000-fold increase in storage density as compared to conventional optical storage schemes. FIG. 2 outlines the wavelength-multiplexing storage scheme based on the PHB phenomenon. When guest molecules such as dye molecules are dispersed in amorphous systems such as a polymer or a rigid glass, they show an inhomogeneously broadened absorption band (band with: .DELTA..omega.ih), which is composed of a lot of homogeneous absorption band of the molecules. (band with: .DELTA..omega.i). This reflects the fact that each guest molecule interacts with amorphous matrix in a slightly different way. At a sufficiently low temperature .DELTA..omega..sub.ih &lt;&lt;.DELTA..omega..sub.i, irradiation to the guest molecules with a light which has a narrow band width results in the selective excitation of the molecules which can resonantly absorb the light in frequency domain. This makes them occur a site-selective photochemistry, so that holes are produced in the inhomogeneously broadened absorption band when the product have a absorption band in another region of frequency domain. The data bits of 1 and 0 are represented by the presence and absence of the hole, respectively.
The hole formation by this method has been reported so far in a variety of organic and inorganic materials (Persistent Spectral Hole-Burning: Science and Applications, edited by W. E. Moerner, published by Springer-Varlag). Most of these systems are based on monophotonic photochemical reactions, such as proton tautomerization and intra- or inter-molecular hydrogen-bond rearrangements.
One of the major problems involved in a material which has a monophotonic hole formation are the absence of a threshold in PHB reaction, so that the destructive readout of stored information cannot be prevented. Moerner and Levenson examined in detail the material parameters required for realizing a sufficient S/N ratio (&gt;26 dB for band width; 16 MHz) under the conditions of high-speed write-in and read-out (30 ns/bit) in a focused laser spot (10 .mu.m.phi.), which are crucial for a practical PHB optical memory (W. E. Moerner and M. D. Levenson, J. of Optical Society of America B, vol. 2, pp. 915 (1985)). They have shown that, for materials which have monophotonic hole formation the allowed region of the material parameters to satisfy the above-mentioned conditions is very limited and that no material thus for reported which has monophotonic hole formation lie within allowed region.
Photon-gated PHB materials were discovered later, in order to prevent the destructive read-out of stored informations, in which two photon photochemical reactions with a threshold are used as a PHB reaction. Photon-gated PHB materials so far reported include, for example, carbazole in boric acid (H. W. H. Lee, et al. Chemical Physics Letters, Vol. 118, pp. 611 (1985)), anthracenetetracene photo adducts in polymethyl methacrylate (PMMA) (M. Iannone, et al. J. of Chemical Physics, vol. 85, pp. 4863 (1986), 8, and the combination of zinc or magnesium tetrabenzoporphyrin and halogenated methanes in PMMA (T. P. Carter, et al. J. of Physical Chemistry, vol. 91, pp. 3998 (1987), W. B. Moerner, et al. Applied Physics Letters, vol. 50, pp. 430 (1987)), as the organic materials; and Sm.sup.2+ in BaCIF (A. Winnacker, Optics Letters, vol. 10, pp. 350 (1985), 7, and Co.sup.2+ in LiGa.sub.5 O.sub.8 (R. M. Macfarlane, et al. Physical Review B, vol. 34, pp. 1 (1986)., as the inorganic materials. Among these, the most promising systems from the viewpoint of applying PHB materials to practical optical memories are those consisting of zinc tetrabenzoporphyrin (TZT) or magnesium tetrabenzoporphyrin (TMT), and halogenated methanes in PMMA due to their high hole-burning efficiency (T. P. Carter, et al. J. of Physical Chemistry, vol. 91, pp. 3998 (1987), and M. E. Moerner, et al. Applied Physics Letters, vol. 50, pp. 430 (1987)). In these systems, where TZT or TMT, and the (1987)). In these systems, where TZT or TMT, and the halogenated methanes are used as a donor and an acceptor, respectively, holes are formed with a two-photon electron transfer from the donor to the acceptor via the triplet state of the donor. By using this reaction scheme, a hole of 1% depth was burnt as fast as in 30ns by means of a CW laser in a focused laser spot of 200 .mu.m.phi.. Furthermore, they could form a hole as fast as in 8 ns, although the laser beams was in a larger spot size of 1 cm.phi. (W. E. Moerner, et al. Applied Physics Letters, vol. 50, pp. 430 (1987)).
These materials, however, have the following drawbacks
(1) The halogenated methanes used as the acceptor are also the solvent for preparing the PHB materials, so that it is very difficult to prepare the medium which has a desired concentration of the acceptor freely with good reproducibility.
(2) The low boiling point of the halogenated methanes used as the acceptor results in an insufficient stability of the medium.
(3) The writing time is not enough fast in view of the duration of the gating light of 200 ms.
(4) The quantum yield of the PHB reaction is still small to realize the practical PHB optical memory.
(5) The gating ratio (i.e., the ratio of the depth of a two-photon hole to that of a one-photon hole with the same irradiation energy) is not large enough to prevent destructive read-out of stored information many times.
(6) The finite lifetime (&lt;100 ms) of the intermediate state of the PHB reaction limits the time interval between the irradiations of the wavelength-selective and the gating light.